The present invention relates to the detection of leaks (including needle-disconnects and other causes of loss of integrity) in extracorporeal blood circuits and more particularly to the application of air infiltration detection techniques to the detection of leaks in positive pressure return lines.
Many medical procedures involve the extraction and replacement of flowing blood from, and back into, a donor or patient. The reasons for doing this vary, but generally, they involve subjecting the blood to some process that cannot be carried out inside the body. When the blood is outside the patient it is conducted through machinery that processes the blood. The various processes include, but are not limited to, hemodialysis, hemofiltration, hemodiafiltration, blood and blood component collection, plasmaphresis, aphresis, and blood oxygenation.
One technique for extracorporeal blood processing employs a single xe2x80x9caccess,xe2x80x9d for example a single needle in the vein of the patient or a fistula. A volume of blood is cyclically drawn through the access at one time, processed, and then returned through the same access at another time. Single access systems are uncommon because they limit the rate of processing to half the capacity permitted by the access. As a result, two-access systems, in which blood is drawn from a first access, called an arterial access, and returned through a second access, called a venous access, are much faster and more common. These accesses include catheters, catheters with subcutaneous ports, fistulas, and grafts.
The processes listed above, and others, often involve the movement of large amounts of blood at a very high rate. For example, 500 ml. of blood may be drawn out and replaced every minute, which is about 5% of the patient""s entire supply. If a leak occurs in such a system, the patient could be drained of enough blood in a few minutes to cause loss of consciousness with death following soon thereafter. As a result, such extracorporeal blood circuits are normally used in very safe environments, such as hospitals and treatment centers, and attended by highly trained technicians and doctors nearby. Even with close supervision, a number of deaths occur in the United States every year due to undue blood loss from leaks.
Leaks present a very real risk. Leaks can occur for various reasons, among them: extraction of a needle, disconnection of a luer, poor manufacture of components, cuts in tubing, and leaks in a catheter. However, in terms of current technology, the most reliable solution to this risk, that of direct and constant trained supervision in a safe environment, has an enormous negative impact on the lifestyles of patients who require frequent treatment and on labor requirements of the institutions performing such therapies. Thus, there is a perennial need in the art for ultra-safe systems that can be used in a non-clinical setting and/or without the need for highly trained and expensive staff. Currently, there is great interest in ways of providing systems for patients to use at home. One of the risks for such systems is the danger of leaks. As a result, a number of companies have dedicated resources to the solution of the problem of leak detection.
In single-access systems, loss of blood through the patient access and blood circuit can be indirectly detected by detecting the infiltration of air during the draw cycle. Air is typically detected using an ultrasonic air detector on the tubing line, which detects air bubbles in the blood. The detection of air bubbles triggers the system to halt the pump and clamp the line to prevent air bubbles from being injected into the patient. Examples of such systems are described in U.S. Pat. Nos. 3,985,134, 4,614,590, and 5,120,303.
While detection of air infiltration is a reliable technique for detecting leaks in single access systems, the more attractive two-access systems, in which blood is drawn continuously from one access and returned continuously through another, present problems. While a disconnection or leak in the draw line can be sensed by detecting air infiltration, just as with the single needle system, a leak in the return line cannot be so detected. This problem has been addressed in a number of different ways, some of which are generally accepted in the industry.
The first level of protection against return line blood loss is the use of locking luers on all connections, as described in International Standard ISO 594-2 which help to minimize the possibility of spontaneous disconnection during treatment. Care in the connection and taping of lines to the patient""s bodies is also a known strategy for minimizing this risk.
A higher level of protection is the provision of venous pressure monitoring, which detects a precipitous decrease in the venous line pressure. This technique is outlined in International Standard IEC 60601-2-16. This approach, although providing some additional protection, is not very robust, because most of the pressure loss in the venous line is in the needle used to access the patient. There is very little pressure change in the venous return line that can be detected in the event of a disconnection, so long as the needle remains attached to the return line. Thus, the pressure signal is very weak. The signal is no stronger for small leaks in the return line, where the pressure changes are too small to be detected with any reliability. One way to compensate for the low pressure signal is to make the system more sensitive, as described in U.S. Pat. No. 6,221,040, but this strategy can cause many false positives. It is inevitable that the sensitivity of the system will have to be traded against the burden of monitoring false alarms. Inevitably this leads to compromises in safety. In addition, pressure sensing methods cannot be used at all for detecting small leaks.
Yet another approach, described for example in PCT application US98/19266, is to place fluid detectors near the patient""s access and/or on the floor under the patient. The system responds only after blood has leaked and collected in the vicinity of a fluid detector. A misplaced detector can defeat such a system and the path of a leak cannot be reliably predicted. For instance, a rivulet of blood may adhere to the patient""s body and transfer blood to points remote from the detector. Even efforts to avoid this situation can be defeated by movement of the patient, deliberate or inadvertent (e.g., the unconscious movement of a sleeping patient).
Still another device for detecting leaks is described in U.S. Pat. No. 6,044,691. According to the description, the circuit is checked for leaks prior to the treatment operation. For example, a heated fluid may be run through the circuit and its leakage detected by means of a thermistor. The weakness of this approach is immediately apparent: there is no assurance that the system""s integrity will persist, throughout the treatment cycle, as confirmed by the pre-treatment test. Thus, this method also fails to address the entire risk.
Yet another device for checking for leaks in return lines is described in U.S. Pat. No. 6,090,048. In the disclosed system, a pressure signal is sensed at the access and used to infer its integrity. The pressure wave may be the patient""s pulse or it may be artificially generated by the pump. This approach cannot detect small leaks and is not very sensitive unless powerful pressure waves are used, in which case the effect can produce considerable discomfort in the patient.
Clearly detection of leaks by prior art methods fails to reduce the risk of dangerous blood loss to an acceptable level. In general, the risk of leakage-related deaths increases with the decrease in medical staff per patient driven by the high cost of trained staff. Currently, with lower staffing levels comes the increased risk of unattended leaks. Thus, there has been, and continues to be, a need in the prior art for a foolproof approach to detection of a return line leak or disconnection.
In an area unrelated to leak detection, U.S. Pat. No. 6,177,049 B1 suggests the idea of reversing the direction of blood flow for purposes of patency testing and access-clearing. The patency tests alluded to by the ""049 patent refer simply to the conventional idea of forcing blood through each access to clear occlusions and to ascertain the flow inside a fistula.
Leaks in the arterial portion of a two-access extracorporeal blood circuit can be very reliably detected because the arterial access line is generally under negative pressure. Thus, any loss of integrity of the arterial circuitxe2x80x94at least those parts under negative pressurexe2x80x94will result in the infiltration of air into the circuit, which can then be detected by an ultrasonic air bubble detector. The air bubble detector is so reliable that, since its introduction in the 1970""s no deaths have been attributed to the failure of this technology to detect a leak. According to the invention, the same technique can be applied to the physical parts of the circuit that are ordinarily under positive pressure by intermittently subjecting them to negative pressure throughout treatment. That is, periodically, the venous side of the circuit is subjected to a negative pressure thereby causing air to infiltrate through any openings in the circuit such as caused by a leak, a loose needle, or a disconnected luer. A conventional air detector, such as an ultrasonic detector, may then be relied upon to detect the loss of integrity and an appropriate response generated. Other technologies, such as radio frequency detection technology, direct audio detection of infiltration using a hydrophone, and others may also be used.
In an embodiment of the invention, the flow of blood from the arterial access to the venous access is periodically reversed. This causes a negative pressure in the venous part of the circuit which, if compromised by a loose or extracted needle or leak, will result in detectable air infiltration. The air infiltration may be detected by the same means used for detecting leaks in the arterial part of the circuit. Advantageously, the flow reversal technique can allow the same leak detection device to be used for detecting both venous circuit leaks and arterial circuit leaks, as long as the flow of blood is controlled in such a way as to insure that any resulting bubbles arrive at the air detector.
To insure that leaks are detected early enough to avoid harm, the period of the flow reversal cycles may be made no greater than a ratio of the maximum allowable blood volume loss to the volume flow rate of the blood. Note that in determining this quantity, due consideration should be given to the fact that some of the patient""s blood is already outside of his/her body in the circuit of the machine. Thus, if the maximum blood loss a patient can tolerate is 500 ml. and the rate of flow of blood is 500 ml./min., the period of the flow reversal cycles may be made no greater than one minute to insure a high level of security.
One technique for reversing the flow in a two-access circuit is to reverse the pump. Another technique is to provide separate pumps, one for each flow direction. At each moment only one pump would run, while the idle pump would either block flow if plumbed as a parallel circuit, or permit flow (e.g., a bypass), if plumbed as a series circuit.
Another technique for providing flow reversal is to employ a four-way valve, which is a four-port valve that transposes the interconnections between the inputs and outputs of the pump and those of the venous and arterial circuits, respectively. Examples of four-way valves that may be used to accomplish this are described in U.S. Pat. Nos. 5,894,011, 5,605,630, 6,177,049, 4,885,087, 5,830,365, and 6,189,388. These prior art valves, however, either suffer from being non-hermetic so that they have seals; are intended to be reused; have stagnant zones, or are simply too expensive.
Preferably, in an ideal four-way valve, the wetted parts form a part of a hermetic disposable unit and have no dead spaces where blood could stagnate. According to an embodiment of the invention, a preferred configuration of a four-way valve defines an H-shaped bridge that is alternately pinched by respective perpendicular anvil-edges to form alternate flow configurations. To form the first configuration, a first anvil-edge pinches the H-shaped bridge at the center of the crossing line of the xe2x80x9cHxe2x80x9d to form mirror image U-shaped channels. In the second configuration, a second anvil edge, perpendicular to the first, pinches the bridge to bisect it and form two parallel channels. Various other hermetic configurations are possible, preferably each provides a four-way valve function and no dead (i.e., no-flow) spaces, which could cause clotting or other deleterious effects, within the valve.
According to another embodiment of the invention, negative pressure in the venous line is generated in just the return circuit. This may be done by periodic paroxysmal expansion of a bladder plumbed inline in the venous circuit. The sudden expansion generates a negative pressure in the venous circuit, drawing air into the blood and exposing any loss of integrity via an appropriately-located air infiltration detector.
The invention will be described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood. With reference to the figures, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.